The conventional technique taught in Patent Literature 1 below discloses a power-factor improving circuit that improves the source power factor to reduce harmonic components contained in the input current. In the power-factor improving circuit, a full-wave rectification mode or a double-voltage rectification mode is selected while the short-circuit start time and short-circuit time of a short-circuit element is controlled in open-loop, so as to realize the functions of improving the power factor and boosting the voltage. That is, the conventional technique of Patent Literature 1 turns on and off a rectifying circuit switch to control a rectifying circuit in the full-wave rectification mode or the double-voltage rectification mode, so that the direct-current output voltage range of the power-factor improving circuit is largely classified into two levels. The area of each of these two levels is further classified, by a variable short-circuiting control of the short-circuit element in the open-loop, into two levels, one without an improved power factor and the other with an improved power factor. Accordingly, the conventional technique provides a total of the four levels of the direct-current output voltage areas, so that the power factor on the high-load side can be improved while the output range of the direct-current output voltage is enlarged.
In the conventional technique shown in Patent Literature 2 below, there is provided a direct-current voltage control unit that outputs a direct-current voltage control signal in correspondence to the deviation value between a direct-current output voltage reference value set corresponding to the load and the voltage across the terminals of a smoothing capacitor, and also there is provided a current reference arithmetic unit that outputs a current reference signal from the product of the control signal from the direct-current voltage control unit and a sine-wave-shaped synchronizing signal synchronous with an alternating-current power supply. By comparing this current reference signal and the current on the alternating-current side of a rectifying element, a switch element is switched on/off at a high frequency to control the direct-current output voltage to be at a desired value while controlling the alternating-current input current to be sine-wave-shaped, so that the source power factor is 1 suppressing the occurrence of harmonics.